The ability to independently control the braking force at each of the vehicle's wheels, together with certain special sensors, enables operation of a vehicle brake system in various special modes of operation. One of these special modes of operation is an anti-lock braking mode of operation, commonly referred to as ABS (for Anti-lock Brake System). Sensors in the vehicle brake system monitor the speed of the vehicle's wheels during braking. If the braking force demanded at a brake for a vehicle wheel causes the wheel to slip, the brake system can momentarily reduce the braking force of the brake at that wheel to allow the wheel to stop slipping, and thus provide optimal braking for the vehicle.
Another of these special modes of operation is traction control. During vehicle acceleration, a vehicle wheel may lose traction, and begin to spin. In the traction control mode of braking, the brake system is electronically actuated, without the driver stepping on the vehicle brake pedal, to individually brake the spinning wheel. When the wheel has slowed sufficiently to regain traction, the brake is released.
In most hydraulic and electro-hydraulic braking systems, solenoid valves are used to control the selective brake pressure in the individual brake lines for both the anti-lock and traction control modes of operation. In a typical integrated anti-lock brake and traction control system, isolation and supply solenoid valves are used to either restrict or facilitate fluid flow from a master cylinder through an individual brake solenoid valve to apply fluid pressure to actuate an individual wheel brake. A dump solenoid valve is generally also provided in conjunction with each individual brake solenoid valve to release the pressurized fluid from the individual wheel brakes and to allow the pressurized fluid to flow to an accumulator. A positive displacement pump then pumps the fluid from the accumulator back through the system to return the fluid to a fluid reservoir for the master cylinder.
During the operation of anti-lock and traction control braking systems, the fluid flowing throughout the system is often subject to pressure and temperature differentials. In particular, a pressure differential generally exists between the fluid pumped from the accumulator back to the master cylinder fluid reservoir. Therefore, an orifice is often used to restrict fluid flow such that pressure is regulated within the system. However, often debris or contaminants can enter the fluid within the braking system, which can subsequently cause the orifice to become clogged and therefore ineffective in its ability to regulate the pressure of the fluid flow in the system.